Aqueous baths and solutions containing complexed heavy metal ions and complexing agents have been advantageously utilized in the electroless deposition of heavy metals, such as copper and nickel, as well as in metal etchant applications. Complexing agents, such as alkanolamines, ammonia or carboxylic acids and their salts form strong bonds or chelates with heavy metal ions, such as copper or nickel, and are used in electroless deposition baths. However, the strong bonding between the heavy metal ions and complexing agents which are advantageously utilized in such applications, present a problem with regard to recovery and/or waste treatment of such plating solutions and the effluents which emanate from processes employing these solutions.
Typically, waste treatment of plating solutions containing heavy metal ions has employed precipitation of the metals, as hydroxides, mainly by addition of lime. Hydroxide formation, however, is prevented by the presence of complexing agents. In situations where complexing agents were present, various precipitation techniques have been employed, such as use of starch xanthate, ferrous sulfate, cellulose xanthate, hydrogen peroxide, sodium hydrosulfate, sodium borohydride, and the like. However, these prior art techniques at best only provided incomplete removal of complexed heavy metal ions and frequently fail to provide acceptable removal of the heavy metals when applied in the field. Also, these materials are expensive, and they produce sludge, which itself requires disposal and does not facilitate reclamation of pure heavy metals.
Thus, an effective and reliable method for selective removal of complexed heavy metal ions from complexing agents has been lacking in the prior art.
It has been suggested that ion exchange resins could be utilized to remove both complexed heavy metal ions and their complexing agents from electroless copper plating solutions, in order to extend the functional life thereof. U.S. Pat. No. 4,076,618 discloses a process utilizing ion exchange resins, primarily cation-exchange resins containing sulfonic acid functional groups. In this prior art process, an electroless copper solution is continuously passed through a series of cation-exchange resins beds, which retain both the complexed copper and its complexing agent together, with the effluent from the bed being discarded. The complexed copper-complexing agent mixture is then removed from the ion exchange resin. Trace amounts of complexed copper and complexing agent which remain after passage through the cation-exchange resin beds are both removed together, by passage through a bed of chelating resin. The complexed copper and complexing agent mixture which is removed from the chelating resin is either returned for use in the electroless copper bath or recovered for subsequent reuse. However, such treatment does not facilitate waste treatment or recovery of pure heavy metal, since the complexed copper and complexing agent are not separated, but remain strongly chelated.
Thus, the process of U.S. Pat. No. 4,076,618 yields substantial amounts of complexed copper and complexing agent mixture which must be processed further for recovery or waste disposal of copper. In addition, the cation exchange resins used have an extremely low efficiency, due to the presence of large quantities of sodium byproducts in the plating solutions or effluents passed therethrough. This results from the greater affinity of cation exchange resins having sulfonic acid functionality groups for alkali or alkaline earth cations, such as sodium, over transition metals, such as copper or nickel. The only utilization of a chelating ion exchange resin in this prior art process is for passage of very dilute solutions containing complexed copper and complexing agents, with the result being removal of the complexed heavy metal and complexing agent together, without separation.
In accordance with the invention, applicants have unexpectedly discovered that a bed of chelating ion exchange resin having an iminodiacetic acid functionality can be utilized to selectively remove complexed heavy metal ions from complexing agents in aqueous solution. In accordance with the invention, applicants provide a method for removal of complexed heavy metal ions from complexing agents, so as to facilitate not only waste treatment and disposal of effluents containing the same, but also recovery of heavy metal ions and control of maximum concentration levels of heavy metal ions in electroless deposition baths containing the same. Accordingly, waste treatment and disposal techniques which utilize biodegradation can then be utilized, whereas previously such was not possible due to the interference with such treatments caused by the presence of complexed metal ions in the aqueous effluents being treated.
In accordance with the invention, applicants provide a simple and inexpensive method for removing substantially all complexed heavy metal ions from solutions in the presence of complexing agents, whereas prior art chemical precipitation methods fail to provide the requisite degree of removal. Furthermore, the method of the invention is efficient, inexpensive, convenient and yields heavy metal in a more concentrated form of high purity, which facilitates easy reclamation or disposal.